1. Field of the Invention
The present invention generally relates to a solid polymer electrolyte membrane fuel cell assembly and a fuel cell stack configuration and, more particularly, to an integrated seal for the same.
2. Description of the Related Art
Electrochemical fuel cells convert fuel and oxidant into electricity. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (hereinafter referred to as the “MEA”) which includes an ion exchange membrane or solid polymer electrolyte disposed between two electrodes typically comprising a layer of porous, electrically conductive sheet material, such as carbon fiber paper or carbon cloth. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane electrode interface to induce the desired electrochemical reaction. In operation, the electrodes are electrically coupled for conducting electrons between the electrodes through an external circuit. Typically, a number of MEAs are electrically coupled in series and/or in parallel to form a fuel cell stack having a desired power output.
The MEA is typically interposed between two electrically conductive bipolar flow field plates or separator plates wherein the bipolar flow field plates may comprise polymeric, carbonaceous, graphitic, or metallic materials. These bipolar flow field plates act as current collectors, provide support for the electrodes, and provide passages for the reactants and products. Such bipolar flow field plates may comprise flow fields to direct the flow of the fuel and oxidant reactant gases to the anode and cathode electrodes of the MEA, respectively, and to remove excess reactant gases and reaction products, such as water formed during fuel cell operation.
Fuel cells need to be sealed in order to isolate the anode and cathode electrodes and to prevent leakage of the reactant gas and product streams either internally inside the fuel cell or externally into the environment. The fuel cell stack typically comprises supply or inlet manifolds for directing the flow of reactant gas streams into the fuel cell stack, as well as exhaust or outlet manifolds for directing the flow of product and excess reactant streams out of the fuel cell stack. Alternatively, the fuel cell stack may comprise coolant inlet and outlet manifolds wherein the coolant is circulated to absorb heat from the exothermic reactions of the fuel cell during operation to maintain the fuel cell stack at a desired operating temperature. These manifolds can be internal manifolds wherein the manifold openings are formed in an extended area of the bipolar flow field plate, or can be external manifolds wherein the manifolds are attached to the edge of the bipolar flow field plate. In a fuel cell stack, the manifold openings of each bipolar flow field plate are in fluid communication with corresponding manifold openings of adjacent bipolar flow field plates to form manifolds thereof for the various fluid streams.
To increase power density of the fuel cell stack, there is a consistent trend to decrease fuel cell stack volume by decreasing the thickness of individual fuel cell components. As the thickness of individual fuel cell components decreases to the micron range, thickness tolerances of individual fuel cell components tend to increase due to manufacturing variability of thin components. Furthermore, when the fuel cell components are assembled to form a fuel cell, the thickness tolerance will increase. Thus, seal design is becoming of greater importance because the seals must be able to withstand a wide range of compression pressure to compensate for the large thickness tolerance of the fuel cell.
MEAs may be individually edge-sealed by sealing around the perimeter of the MEA prior to fuel cell stack assembly. One method is to attach a sheet-type gasket frame around a perimeter of the MEA. Elastomeric seals are formed on the bipolar flow field plates and compressed against the gasket frame under a compression pressure, thus providing a substantially fluid leak-tight seal and thereby isolating the reactant gases and product streams and their corresponding inlet and outlet manifolds. However, this sealing method is not cost-effective because it requires a number of materials to form a substantially fluid leak-tight seal. Moreover, variations in MEA thickness around the perimeter of the MEA may result in non-uniform pressure exertion by the reaction force produced by the elastomeric gaskets, and therefore non-uniform sealing may occur around the perimeter of the MEA.
Another method of edge-sealing MEAs is disclosed in U.S. Pat. No. 6,699,613. A liquid sealant is directly in contact with the projecting portion provided at the periphery of the solid polymer electrolyte membrane, is pressed between the solid polymer electrolyte membrane and the separators, fitting the varying sizes of the seal sections, and maintains gas-tightness between the solid polymer electrolyte membrane and the separators (hereinafter referred to interchangeably with “flow field plates”). However, this approach is problematic because the MEA is adhesively attached to both separators and cannot easily be removed from them without damaging the MEA and/or the separators. Thus, if an MEA is degraded and needs to be replaced, the separators will also need to be replaced, thereby increasing replacement costs.
A similar approach is described in U.S. Pat. App. No. 2004/0168306, which discloses a method of laminating a separator and a membrane/electrode assembly for fuel cells and an apparatus for laminating the same. This method corrects a warp in a separator applied with a sealant during production of fuel cells. The correction is performed at a correcting device. With the warp being corrected at the correcting device, a membrane/electrode assembly is superimposed on the separator. Since the membrane/electrode assembly is superimposed on the separator while the separator is corrected with the correcting device being operated, the sealant applied to the separator can be spread out to an even thickness, providing good sealing. However, this method also results in an MEA that is glued to both plates and, thus, entire fuel cell assemblies would need to be replaced when replacing degraded MEAs.
MEAs can also be individually edge-sealed with silicone-based elastomers that are injection-molded to encapsulate and/or impregnate the perimeter of the electrochemically active area of the MEA. However, silicone-based elastomers have been shown to degrade under certain fuel cell operating conditions and exhibit creep and compression set under prolonged stack compression, which can lead to seal thinning with extended fuel cell operation as well as internal and external leakage of the reactant gas and/or coolant. Furthermore, the polymer electrolyte membrane at the perimeter edge of the MEA may experience membrane thinning, thereby increasing the occurrence of premature membrane failures.
U.S. Pat. App. No. 2004/0161655 discloses a method for assembling electrochemical cells for monopolar arrays or bipolar stacks using an adhesive to bond and seal the interfaces of the stack components. Accordingly, no gaskets, o-rings or similar devices are required to seal between the components. However, this method is also undesirable because it would be difficult and expensive to replace individual fuel cell assembly components that have degraded because all the fuel cell assemblies and components are adhesively attached together in the fuel cell stack.
Given these problems, there remains a need to improve the sealing design of fuel cells to improve durability, and to decrease cost and complexity. The present invention addresses these issues and provides further related advantages.